Re: [Caml-list] Question on "more general"

[Ivan Gotovchits <ivg@ieee.org>]

[-- Attachment #1: Type: text/plain, Size: 1883 bytes --]

A small update.

But before the update, let me clarify the original reason, why the
expression `let a = assert false in fun x -> 1` is not generalized. The
reason is because the `assert false` is not nonexpansive (sorry for the
double negation). A nonexpansive value, in OCaml compiler parlance, is a
value that has "no _observable_ side effects". The `assert false` is
currently typed as a special expression, that is assumed to have a side
effect (i.e., it is expansive).  Thus, it is basically treated the same as
`let a = print_int 1 in fun x -> 1` that is also not generalized, as the
evaluation of the `print_int 1 in fun x -> 1` has some effect.

Apparently, the assumption that `assert false` is expansive was too
conservative, as it was already subsumed (in some cases) with the value
restriction relaxation. So it will be relaxed soon, see GPR#1142 [1]. And
soon both of your functions will have the same type.



[1]: https://github.com/ocaml/ocaml/pull/1142

On Thu, Apr 20, 2017 at 11:13 AM, Kenichi Asai <asai@is.ocha.ac.jp> wrote:

> Thank you!  That's very clear.  So instead of writing assert false, if
> I could write a "value" of type 'a, then the type of x becomes 'a.
>
> I confirmed it with the following file:
>
> --------------------------------
> let v = assert false
>
> let g1 = let a = v in
>          let f = fun x -> 1 in
>          f
>
> let g2 = let a = assert false in
>          let f = fun x -> 1 in
>          f
> --------------------------------
>
> If I see the types of variables via "ocamlc -i", I obtained:
>
> val v : 'a
> val g1 : 'a -> int
> val g2 : '_a -> int
>
> Thank you again!
>
> --
> Kenichi Asai
>
> --
> Caml-list mailing list.  Subscription management and archives:
> https://sympa.inria.fr/sympa/arc/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
>

[-- Attachment #2: Type: text/html, Size: 2929 bytes --]